There has conventionally been known such a centrifugal fan that is used as a centrifugal fan and has an orifice different from a suction casing with a bell-mouthed suction port at an opening on one side of an outer casing. An example of such a centrifugal fan in the related art is described in Patent Document 1.
The centrifugal fan in the related art will be described below with reference to FIG. 31.
As illustrated in FIG. 31, the centrifugal fan in the related art has outer casing 101 having one opened side, motor 104 that has impeller 103 fixed to top surface 102 in outer casing 101, casing 105 that surrounds a periphery of impeller 103, and suction casing 107 having suction port 106. Orifice 109 has suction hole 108 having a diameter equal to or smaller than that of suction port 106. Orifice 109 is separated from suction casing 107 by predetermined clearance h. Orifice 109 configures resonant space 112 so that lower end 110 of suction casing 107 and end 111 of orifice 109 are separated by clearance distance i. Grill 113 is provided on one side of orifice 109. Discharge port 114 is provided on one side of outer casing 101.
In the above configuration, when impeller 103 is rotated, sucked air passes from grill 113 through suction hole 108 of orifice 109 and then enters impeller 103 from suction port 106 of suction casing 107. The sucked air is subjected to pressure rise by impeller 103. The sucked air passes through an inside of casing 105 and is then discharged from discharge port 114. Sound waves of rotational noises caused when the sucked air is subjected to pressure rise by impeller 103, vortex turbulent noises caused when the sucked air passes through casing 105, and noises caused in casing 105 are emitted from suction port 106. Some of them are incident from inlet portion 115 having clearance distance i into resonant space 112. The incident sound waves of the noises at a frequency specified according to a volume and shape of resonant space 112 are resonated and suppressed in such a manner that air column resonance occurs in resonant space 112 and that inlet portion 115 and resonant space 112 function as a Helmholtz resonator.
The frequency of the noises that are resonated and suppressed in resonant space 112 is specified according to the volume and shape of resonant space 112. In such a centrifugal fan in the related art, a range in which the volume and shape of resonant space 112 can be adjusted is small. For this reason, a range of frequencies of noises that can be resonated and suppressed is small. The range of frequencies of noises that can be suppressed is therefore required to be increased.
To suppress noises at a low frequency, the volume of resonant space 112 needs to be increased. A size of outer casing 101 thus becomes larger. Noises at a lower frequency are required to be suppressed without changing the size of outer casing 101.
The volume of resonant space 112 cannot be easily changed. If main frequencies of noises are changed according to an installed state of the centrifugal fan or there are noises at a plurality of outstanding frequencies, a noise reduction effect by resonance noise-suppression may be reduced. Frequencies at which noises can be suppressed are required to be adjusted.    [Patent Document 1] Japanese Patent No. 3279834